This invention relates to control circuits for rotary tools. More specifically, it relates to circuits which control the operation of a torquing tool as, for example, a nut runner or similar device. These type of torquing tools may be electrically or pneumatically powered and are utilized for drawing a nut down upon a threaded bolt or similar torquing operations on other types of fasteners.
The devices, per se, have been in use for some period of time. Determining the operation period of the device for each fastener has long been a problem in order that maximum tightness and strength can be achieved. Accordingly, most tools include means for limiting the amount of torque which is developed by the tool so that the fastener tightened by the tool is not torqued to a point where it cracks or is significantly weakened. Prior techniques of controlling the shut off point of the device include the use of a slip clutch, a torque sensing device to compare developed torque against a preset maximum and similar arrangements.
Recently, it has been determined that maximum fatique resistance with minimum added stress can be developed on a fastener by tightening a nut onto a bolt to a point where the bolt (or other fastening device) first enters yield or slightly beyond depending upon the desired application. For a further discussion of this subject reference is made to the following article: "Preloading for Optimum Bolt Efficiency", R. J. Finkelston, August 1974, Assembly Engineering, pp. 24-28.
At present no device has been developed which can accurately detect the onset of yield and thereby control torque applied to a fastener during the operation of the nut runner or similar device. Examples of prior art devices which include torque control or torque sensing include U.S. Pat. Nos. 3,827,506 and 3,538,763. In both disclosures the fasteners are torqued to a preselected maximum torque without regard to the torque necessary to cause a given fastener to enter yield. Thus, sample to sample variations in the fasteners produce less than optimal results in that some fasteners are torqued into yield while others may not be or are over torqued wearing them.
One known method of determining the onset of yield is to measure the change in torque with respect to the angular position of the tool. While this is possible it has the distinct disadvantage that the tool must be mounted in a frame or other fixture so that the angular orientation of the tool does not shift and thereby uncalibrate the control circuit.
This drawback is overcome in the present invention in which the onset of yield is detected by measuring the change in torque with respect to time. This is possible because time is clearly related to the revolutions of a torquing tool if the tool has sufficient power to shear the bolt on which it is driving a fastener. Freeing the tool from a fixture is an important requirement if such tools are to be widely used in present day assembly operations.
It is accordingly an object of the present invention to provide a torquing tool control circuit which is capable of detecting the onset of yield in a fastener and shutting off the torquing tool at a selected point in the plastic region.
It is another object of the invention to provide a control circuit which will cyclically operate the torquing tool to prevent excessive torque rates as the fastener approaches the yield point.
It is another object of the invention to provide a control circuit for a torquing tool which can detect the onset of yield in a fastener without the use of angular encoders or the requirement of a fixturing device for the tool.
A further object of the invention is to provide a pneumatic nut running system in which a torquing tool is closely controlled to repeatedly achieve a desired torque in the elastic range of the fastener.
These and other objects of the invention will become apparent from the remaining portion of the specification.